A compression driver comprises a pole piece made of ferromagnetic material which has a bore therein, the front end or opening of which is adaptable for coupling to the throat of a horn. A diaphragm, usually circular with a central dome-shaped portion, is mounted adjacent the rear opening of the bore so as to be freely vibratable. Attached to the edge of the diaphragm's dome is a cylindrical coil of wire, the voice coil, oriented so that the cylindrical axis of the coil is perpendicular to the diaphragm and coincident with the axis of the pole piece bore. A static magnetic field, usually produced by a permanent magnet, is applied so that an alternating signal current flowing through the voice coil causes it to vibrate along its cylindrical axis. This in turn causes the diaphragm to vibrate along the axis of the bore and generate sound waves corresponding to the signal current. The sound waves are directed through the bore toward its front opening. The front opening of the bore is usually coupled to the throat of a horn which then radiates the sound waves into the air. In the description that follows, the term "throat" is used to mean either the front or downstream end of the pole piece bore or the actual throat of a horn. Interposed between the diaphragm and the pole piece bore is a perforated structure known as a phasing plug for impedance matching the output of the diaphragm to the horn. Within the phasing plug are one or more air passages or channels for transmission of the sound waves. The surface of the phasing plug opposite the diaphragm is of corresponding sphericity and positioned fairly close to the diaphragm while still leaving an air gap, or compression region, in which the diaphragm can vibrate freely.
The phasing plug effects two basic functions. First, because the cross-sectional area of the air channel inlets are smaller than the area of the diaphragm, the air between the diaphragm and the phasing plug (i.e., the compression region) can be compressed to relatively high pressures by motion of the diaphragm. This is what allows a compression driver to output sound at greater pressure levels than can conventional loudspeakers where the diaphragm radiates directly into the air. The efficiency of the loudspeaker is thus increased by virtue of the phasing plug being placed in close opposition to the diaphragm to minimize the volume of air between the diaphragm and the phasing plug Secondly, as the name "phasing plug" implies, the path lengths of the air channels within the phasing plug may be equalized so as to bring all portions of the transmitted sound wave into phase coherence when they reach the throat. Without such path length equalization, sound waves emanating from different air channels would constructively or destructively interfere with one another at certain frequencies so as to distort the overall frequency response.
Phasing plugs have been made with many designs. Perhaps the most frequently used type is one having annular cross-sections that usually increase in area as the principal radius of each annulus decreases in moving toward the throat of a speaker. This is shown, for example, in U.S. Pat. No. 2,037,187, entitled "Sound Translating Device," issued to Wente in 1936 and hereby incorporated by reference. Another type is the saltshaker design, so called because holes at the spherical outer surface of the plug that extend through to the throat of the speaker resemble the holes of a saltshaker. Another design that has been used, shown in U.S. Pat. No. 4,050,541, entitled "Acousticla Transformer for Horn-type Loudspeaker" and hereby incorporated by reference, couples the diaphragm region to the throat by radial slots extending from the axis of cylindrical symmetry of the speaker.
In order to provide a low reluctance magnetic pathway for the applied static magnetic field, the permanent magnet and the voice coil are disposed within a surrounding environment of ferromagnetic material. As both the magnet and voice coil are commonly located on the side of the diaphragm facing the pole piece, the magnetic pathway includes both the phasing plug and the surrounding pole piece. In order for the voice coil to be free to vibrate, however, it must be disposed within an annular air gap which will be referred to herein as the coil space. Ideally, the coil space should be made as small as possible since air in the magnetic pathway adds reluctance to the magnetic circuit which lessens the field strength at the voice coil. Nevertheless there is a considerable volume of air in the coil space surrounding the voice coil as well as in the spaces along the inner edge of the surround and outer edge of the diaphragm which are continuous with the coil space. This region, comprising the coil space and the space along the surround and outer edge of the diaphragm, is thus an uncoupled region since it is so far from the inlets of the phasing plug air passages that variations of air pressure in that region are coupled little or not at all to the phasing plug and thence to the throat. Such an unused volume is shown in the Wente patent referred to above. These pressure variations thus result in energy losses which lead to heating of the loudspeaker but do not result in the generation of useful sound output. The uncoupled region also causes cavity resonance effects which distort the overall sound output of the speaker due to anomalies in its frequency response. Such resonances, known as parasitic resonances, present a significant design problem for the speaker designer. (See, e.g., "The Influence of Parasitic Resonances on Compression Driver Loudspeaker Performance" by Kinoshita, et al. presented at the 61st Convention of the Audio Engineering Society in 1978 and available as preprint no. 1422 (M-2).)
It would be useful to couple the pressure variations in the uncoupled region around the voice coil to the throat of the horn, in addition to the pressure variations produced by the diaphragm, to improve the efficiency and sound quality of the loudspeaker. Use of the additional pressure variations could be expected to reduce heating in the region around the voice coil as a result of repeated compression and rarefaction of the same air in that region, to produce an increase in the efficiency of the loudspeaker, and to reduce parasitic resonances.